The reduction of emission of NOx generated by combustion processes is of increasing importance. Technologies that seek to reduce NOx by modifying combustion conditions and fuels are known as primary measures, while technologies that treat the exhaust resulting from combustion are known as secondary measures.
Among secondary measures are selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR), both of which are conducted with ammonia and urea as reducing agents. The direct use of ammonia in these processes is difficult, because of safety problems associated with its storage, handling, and transport. Direct use of urea is safer, but there are high energy requirements for converting urea from a solid or aqueous form into an active gaseous species.
A method of selective catalytic reduction of NOx using a side stream of combustion flue gas is described in U.S. patent application Ser. No. 11/275,989, filed on Feb. 8, 2006, and published on Jun. 1, 2006, as U.S. Patent Application Publication No. US2006/0115402, the entirety of which is incorporated herein by reference. The described method seeks to overcome the difficulty of converting urea to ammonia for use in an ammonia-SCR NOx treatment process by making use of a side stream (e.g. slipstream) of the combustion flue gas to be treated. This side stream, generally no more than about 3% of the total flue gas to be treated, is directed through a channel into which urea is injected. The side stream is heated so that the injected urea is decomposed, and the reaction products are carried with the side stream of flue gas into an injection grid of an SCR unit. The method achieves uniform mixing of active gaseous reactants for NOx reduction by SCR.
A disadvantage of this process is its thermal inefficiency. Conventional heat exchangers (e.g. as shown in US2006/0115402 at reference 22 of FIGS. 1-11) often do not provide sufficient heat for conversion of urea to ammonia. This necessitates the use of a burner to directly heat the side stream (e.g. as shown in US2006/0115402 at reference 38 in FIGS. 2-6 and 11). The burner consumes fuel and is inefficient, since the heat it creates is not harnessed for later use.
There is a need for a more thermally efficient process for converting urea to ammonia in a side stream of combustion flue gas for catalytic reduction of NOx via SCR.